1. Field of the Invention
The present invention relates to a sensor for non-invasive and continuous determination of the duration of arterial pulse waves.
2. Description of the Related Art
A desirable goal for many manufacturers of blood-pressure measuring devices is continuous non-invasive measurement of human blood pressure without the aid of uncomfortable compression cuffs.
It has been known for quite some time that human blood pressure, differing from individual to individual, correlates to the velocity of pulse waves. Hitherto this fact could not be exploited for continuous measurement of blood pressure, because there are no reliable, inexpensive sensors available for determination of the pulse-wave velocity.
Hitherto, attempts have been made to determine the pulse-wave velocity, which is about 10 m/s at the wrist, via the change in the color, the form or in the electric resistance of the skin. Attempts have also been made to measure the pulse-wave velocity with the aid of the ultrasonic Doppler method, a reliable but not exactly inexpensive method. One such attempt is described in DE-OS-1 905 620. Two spaced apart piezoelectric oscillator systems, the conical-shaped sound of which irradiates a to-be-examined vessel and a Doppler reception device permit determination of the vessel-wall velocity with which the vessel wall is extended by the blood-pressure waves flowing through the vessel. Ultimately information on the pulse-wave velocity is obtained via a special evaluation algorithm.
Another example for determining the pulse-wave velocity is indicated in U.S. Pat. No. 4,245,648 describing a process and a device for measuring blood pressure and for determining the pulse rate. Two pressure-sensitive sensors housed in an arm cuff are placed along a blood-conducting vessel. The increase-in-pressure values determined in intervals can be utilized for calculating the pressure-wave velocity. However, disadvantageous is the large size of the device making it impossible to use at sites that are difficult to reach. Furthermore, application of the device involves considerable motoric impediment.